But in a statement responding to the announcement, he gave no indication that the UK is considering an increase to its 500-strong deployment,

His statement also stressed that an additional 85 troops were committed as recently as June.

Sir Michael was briefed by his US counterpart, Jim Mattis, ahead of the president's announcement, in the first of a series of phone calls to Nato allies. London has not yet received any formal request for military support.

The Defence Secretary said: "The US commitment is very welcome. In my call with Secretary Mattis yesterday we agreed that, despite the challenges, we have to stay the course in Afghanistan to help build up its fragile democracy and reduce the terrorist threat to the West.

"It's in all our interests that Afghanistan becomes more prosperous and safer: that's why we announced our own troop increase back in June."

Mr Trump said his reversal in opinion had come after studying Afghanistan from "every conceivable angle". He added:

"My original instinct was to pull out and historically I like following my instincts, but all my life I've heard that decisions are much different when you sit behind the desk in the Oval Office."

He said he would not put a timescale on the end of the war, instead waiting until they can properly survey conditions on the ground in Afghan.